1. Field of the Invention
The present invention relates generally to remote control of downhole tools through pressure change signals transmitted through a column of fluid in the well.
2. Description of the Prior Art
Traditionally, downhole tools such as those utilized in drill stem testing of oil and gas wells have been controlled either by physical manipulation of the pipe string which carries the tools or by changing the pressure applied to a column of fluid standing in the well, with that pressure being directly mechanically applied to a power piston of the tool so as to move an operating element of the tool. This second mode of operation includes those tools which are directly operated by changing well annulus pressure which is communicated with a power piston of the tools, or so-called annulus pressure responsive tools. The operating pressures for annulus pressure responsive tools typically are relatively high, on the order of 1,000 to 1,500 psi above the hydrostatic pressure of the well annulus fluid.
More recently, the development of downhole tools including programmed electronic controllers has made possible the use of remote controlled tools which may receive command signals transmitted from a remote command station, located at the earth's surface, through any one of several means to a receiver contained in the tool. The programmed electronic controller then causes the operating element of the tool to be actuated through any one of several types of operating systems in response to the remotely received command signal.
One system which has been developed for remote communication with such a preprogrammed remote control downhole tool is that set forth in Upchurch U.S. Pat. Nos. 4,896,722; 4,915,168; 4,796,699; and 4,856,595. The Upchurch patents propose a communication system utilizing one or more low level pressure pulses transmitted through a column of well annulus fluid. The Upchurch patents define low level pulses as being those less than or equal to 500 psi above hydrostatic well annulus pressure. The pulse proposed by Upchurch includes a rise from hydrostatic pressure to the low level peak at some value below 500 psi, with that low level peak being maintained for a period of time which determines the informational content of the pulse, and the pressure then dropping back to hydrostatic pressure. An operative command signal signature for a downhole tool utilizing the Upchurch system may include one or more of these low level pulses.